Mast cells arise from HSCs from 48 to 72 hpf. Two morpholinos targeting the zebrafish runx1 gene were injected in 1 to 4 cell stage wild-type embryos followed by WISH for cpa5. runx1 morphants demonstrate a mild decrease in the number of cpa5-positive mast cells at 48 hpf (n = 28) and a progressive decrease at later time points, 60 hpf (n = 14) and 72 hpf (n = 41), compared with wild-type embryos (n = 31, 16, and 17, respectively), implicating that HSCs contribute to the production of the definitive mast cell lineage (5× objective, MZ6 microscope; Leica). Insets: Higher magnification views of the anterior of the embryos. Bar graphs represent the percent of embryos categorized by WISH expression levels. MO indicates morpholino.